The invention relates to ion sensitive field effect transistors (ISFETs), and more particularly, to methods for fabricating SnO2 extended gate ISFETs.
The ion sensitive field effect transistor (ISFET) was presented by Piet Bergveld in 1970. An ISFET with reference electrode is similar to Metal-Oxide-Semiconductor Field Effect Transistor (MOSFET), except that the ISFET has exposed the gate insulator to measure a selected ion concentration in electrolyte. When the pH-ISFET is immersed in an aqueous solution, a surface potential is induced at the surface of the detection membrane of the pH-ISFET. However, the surface potential at the sensing membrane will affect the carrier concentration within the inversion layer of the semiconductor, due to the gate dielectric layer being extremely thin. Thus, the current, which flows through the channel, is adjusted. Furthermore, the surface potential is related to the hydrogen ion activity within the aqueous solution. As the pH values change, different surface potentials are induced at the detection membrane, leading to different channel currents. Thus, the pH-ISFET can be used to detect the pH values of solution.
Further, the extended gate field effect transistor (EGFET) structure was presented by J. V. D. Spiegel et al, in which the detection film is extended from the gate terminal of the field effect transistor by a conductive line. Thus, only the detection film requires immersion in a testing solution, without the field effect transistor.
A variety of materials are known to be capable of serving as ISFET detection film, such as, Al2O3, Si3N4, a-WO3, a-C:H, and a-Si:H, etc. The manufacture of detection films is typically accomplished by deposition, such as, sputtering or plasma enhanced chemical vapor deposition (PECVD). Thus, the cost is relatively high and the time required for thin film fabrication is excessive.
Thus, an easily fabricated, low cost ISFET and the detection film thereof, eliminating packaging problems, are desirable.